GOODYEAR, Ariz. -- The only thing the Reds could see from the dugout on Monday was that Aroldis Chapman wasn't himself on the mound. They were surprised when they learned the reason why.

Chapman, the club's prized Cuban signing, could not finish his outing vs. the Rockies because of a stiff lower back. Slated to work four innings of relief for Bronson Arroyo, Chapman lasted just 1 2/3 innings.

The club later diagnosed Chapman as having muscle spasms, and said he is getting treatment and will be evaluated again later in week.

"I'm not really hurt, but I have a little problem with my back that I've had all week," Chapman said with Class A Dayton pitching coach Tony Fossas translating.

That was news to Reds manager Dusty Baker and pitching coach Bryan Price.

"He didn't say anything," Baker said after the Reds' 9-1 loss to Colorado. "I guess in Cuba, you're taught not to complain or say much. We could tell he was wincing and walking around the mound. He really didn't have his same stuff -- same fastball or anything. We could tell something was wrong."

What this might do to the battle for the fifth starter's spot in unknown. Chapman was slated to see the doctor after leaving Goodyear Ballpark.

"Let's not speculate until we find out what's wrong," Baker said.

Chapman gave up four unearned runs with two hits, two walks and a wild pitch. Overall, he threw 38 pitches, including 21 for strikes. The outing began with a perfect, eight-pitch top of the sixth inning and he topped out at 97 mph on his second pitch to Troy Tulowitzki.

"The first inning I felt it a little bit and was able to work through it," Chapman said. "Then it got stiffer as the game went on."

Normally able to reach 98-100 mph, Chapman did not have his usual zip. On his first pitch of the seventh inning to Brad Eldred, he hit the backstop.

"We're not going to send guys out there that have problems or ailments that we're aware of," Price said. "Sometimes guys are going to go out there and compete because of the lure of trying to make the team and things of that nature. If this was an issue, then he shouldn't have been pitching. Hopefully, next time we'll know before he gets to game time."

In the top of the seventh, Eldred led off with a sharply lined single to left field. After a fielder's choice, Cole Garner stole second when Chapman failed to hold the runner. Pinch-hitter Jonathan Herrera walked on a 79-mph changeup in the dirt.

Trouble continued when Dexter Fowler's potential double play ball went off shortstop Drew Sutton's glove for an error that scored Garner. After Eric Young struck out with the count full, Alex Escobar walked and loaded the bases.

That brought Price out of the dugout for the first time.

"It was a regular, typical mound visit," Price said. "I just wanted to remind him that he's got a good fastball and to utilize it more."

Jordan Pacheco laced a three-run double to left field and made it a 7-1 game. Chris Nelson reached a 2-1 count, including a wild pitch, when Baker, Price and the trainer visited the mound and removed Chapman.

"We went out there and kind of had to pull it out of him. First he said he was all right," Baker said. "I was like, 'Now you have to tell the truth. Something's wrong.' We don't think it's anything serious. We took him out before it got worse."

A sore back in Spring Training isn't a unique injury for a pitcher, but this isn't your average pitcher.

The Reds created quite the buzz in January when Chapman, a Cuban defector, was signed to a six-year, $30.25 million contract. All of his outings have drawn national media attention and it just so happened that Monday's game was carried live on local television and MLB Network.

Now 22 years old and the top contender for the fifth spot in the big league rotation, Chapman has dazzled during camp not only with his triple-digit velocity, but better-than-advertised command of secondary pitches like his slider and changeup.

Through four games, including one start, Chapman has a 1.04 ERA in his 8 1/3 innings with four walks and a team-leading 12 strikeouts. He is considered part of a seven-pitcher race for the fifth spot, but the battle seems to be most competitive among him and two other contenders -- Travis Wood and Justin Lehr.

It wasn't known how many days Chapman would be unable to throw.

"I certainly don't want anybody to have any problems -- arm problems, back problems or ailments -- that will keep them from competing on the field," Price said. "We'll just wait and see. We're just kind of finding out about this. We want to make sure it doesn't exacerbate the problem by pitching and set him back. Hopefully, it's a couple of days of treatment and he's back on the mound competing again."